Stencil and apparatus for supporting and tensioning the stencil

ABSTRACT

A support and tensioning frame for a stencil to enable solder paste to be applied to printed circuit boards and other electronic substrates includes a support frame engaged with at least two opposed mounting members. Each mounting member includes at least one projection for engaging an aperture in a stencil to mount the stencil under tension. A permanent-biasing mechanism engages one or more of the mounting members to place the stencil under tension. A second biasing mechanism engages the mounting member to provide a bias opposite that of the permanent-biasing mechanism to facilitate release of a mounted stencil.

RELATED APPLICATIONS

This application is a continuation of prior U.S. application Ser. No.09/586,824, filed on Jun. 5, 2000, which is a divisional of U.S.Application Ser. No. 09/483,253 (now U.S. Pat. No. 6,289,804), filed onJan. 13, 2000, which is a continuation of U.S. Ser. No. 08/983,621 (nowU.S. Pat. No. 6,067,903), filed on Jun. 1, 1998. Additionally, like thepreceding applications, this application claims the benefits of both PCTApplication PCT/GB96/01722, filed Jul. 18, 1996, and GB9514927.4, filedon Jul. 20, 1995. The entire teachings of each of the above-listedpriority applications are incorporated herein by reference.

BACKGROUND

In GB 2264460 of Micro Metallic Limited, there is disclosed a supportand tensioning frame and a rectangular stencil mountable thereon whereinat least two opposite edge regions of the stencil have apertures whichreceive displaceable mounting fingers or mounting teeth on oppositesides of the frame and tension the stencil by rotary or lineardisplacement of the teeth which abut the edges of the stencil and lieout of the main plane of the stencil in the tensioned condition.

In GB 2292115A of N. M. Baker, a mounting frame is disclosed whereinsprings bias stencil mounting bars inwardly into a non-tensioningposition and manually operable levers are used to displace cams to moveat least two adjacent mounting bars outwardly to tension the stencil.Manual operation is required and the cams may not apply the same eventension nor the same force nor is there the same ease of stencilmounting as in the present invention and the connection means of thisprior application extend into the plane of the main body of the stencil.

SUMMARY

A support and tensioning frame for a stencil for enabling solderingpaste to be applied to electric circuit boards comprises a rectangularsupport frame with each of at least two opposite side regions of theframe having an elongate stencil support surface against which a stencilabuts and is flexed in such region on tensioning, and a displaceablemounting bar carrying a row of projections or teeth that are insertableand engageable in apertures in opposite side regions of a stencil. Themounting bars of said at least two opposite sides of the frame arepermanently biased, for example, by spring means, to move away from eachother into the stencil engaging position of the teeth. Optionallyoperable displacing means are provided for optionally displacing eachsaid mounting bar against the action of its biasing means to displacethe mounting bars into closer proximity and, in use, to displace thetensioning teeth out of the apertures in the opposite side regions ofthe stencil to permit the stencil to disengage from the teeth and frame.

For achieving most accurate stenciling results, a rectangular (e.g.,square) stencil can be tensioned along all four sides; the followingdescription will mainly be in respect of a stencil havingteeth-receiving apertures along all four sides thereof and to asupporting and tensioning frame having mounting bars and tensioningteeth along all four sides thereof.

The frame can operate such that the mounting means, which can be in theform of teeth, do not extend into the plane in which the main centralbody of the stencil lies when tensioned, which means the edge regions ofthe stencil have to be flexed over/under surfaces of the frame to beengaged by the mounting teeth in this embodiment.

The optionally operable displacing means can accordingly be providedalong all four sides of the frame and can be simultaneously operable andcan comprise an inflatable and deflatable tubular or other flexibleelongate member (normally closed at one end), which can extendcontinuously around the four sides of the frame so as to act on the fourdisplaceable mounting bars equally and simultaneously. A releasablevalve means can be provided to optionally maintain the tubular means inthe inflated condition prior to tensioning of the stencil and until thevalve means is released to cause deflation of the tubular means via thebiasing means to enable the teeth to engage in the apertures in edges ofthe stencil. The displacing means can be pneumatically inflatable.

The supporting and tensioning frame can be formed as a continuouschannel formed of side sections with corner connecting blocks and withside sections of substantially U-section with the outermost limb or wallbeing of a shorter length than the inner limb or wall to facilitatemounting of the stencil—as will be subsequently described.

Also, a stencil support and mounting means for facilitating theengagement of teeth of at least two opposite side regions of arectangular supporting and tensioning frame in apertures in at least twoopposite edge regions of a stencil and for facilitating the depositionof said stencil, comprises a base means form which there extend at leasttwo spaced apart projections providing support surfaces for engagementwith at least two opposite edges of a rectangular or square stencil whenthe stencil is located extending therebetween such that the stencil isfreely supportable and downwardly flexible, and guide and location meanscooperable with surfaces of a rectangular or square supporting andtensioning frame for the stencil which when said stencil is supported onsaid projections and said frame is being guided and located seating onsaid support means, said projections restrain downward deflection of theedge regions of the stencil which is otherwise flexed against anelongate support surface of the frame, and the teeth in said at leasttwo opposite side regions enter into the apertures in the at least twoopposite side regions of said stencil and such that on release ofoptionally operable displacing means of said frame, said teeth arebiased to be displaced outwardly to enter in said apertures and engagethe stencil and hold the stencil in a tensioned condition.

Also, according to one aspect of the invention, there is provided acombination of frame, stencil and stencil support.

The base means can comprise a rectangular and preferably square sheet,and the at least two projections and the location and guide meansthereof are provided by at least two spaced apart stepped membersextending along opposite side regions of the base sheet with the innernormally horizontal surface of the steps of at least two facingprojections acting to support opposite edges of a stencil and outersubstantially vertical or upper extending surfaces of the projectionsacting to locate and guide the supporting and tensioning frame. Fourlocation and guide means can be provided—one along each side so that aprecise location of the teeth within the apertures in the stencil isachieved. Other arrangements, such as a plurality of spaced apartprojections and/or corner location/guiding for the frame, may beprovided.

The stencil support and mounting means can comprise a rectangular basesheet having four elongate stepped supports each extending along a sideof said sheet and each having a shoulder on its inner side forsupporting the edge of a stencil above the surface of the base sheet;said stepped supports being shaped and dimensioned and located so that astencil support and tensioning frame may locate thereover and preferablyaround although if the projections are two-part, preferably within theouter part, so that teeth of the frame align with slots of a stencilsupported on the shoulders of the support and mounting means.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described further, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is an inverted schematic plan view of a square supporting andtensioning frame for supporting a stencil (2) but primarily showing thechannel (3) formation of the main part of the frame which is ofsubstantially U-shaped section, and also showing, in the lower edge, adisplaceable mounting bar (4) (which is one of four—one each side of theframe 1), whilst in the upper left-hand quadrant (5), a quarter part ofa stencil (2) is illustrated, namely an upper quarter part of thestencil (2) having a plurality of parallel elongate slots (6) thereinand extending along all four sides of the stencil (2) for receivingteeth (7) of the frame (not shown in FIG. 1 and only one shown in FIG.2);

FIG. 2 is an enlarged fragmentary schematic section through one side ofthe stencil supporting and tensioning frame (1) and with the U-shapedchannel (3) including the displaceable mounting bar (4) carrying a rowof teeth, spring biasing means and an optionally inflatable displacingmeans together with the projection of a stencil supporting means;

FIG. 3 is a plan view of a stencil support and mounting meansincorporating the projection and location and guide means illustrated insolid black in FIG. 2 and provided along four sides of a base means orsheet;

FIG. 4 is an elevation of one projection means and location and guidemeans of one side and which is located by being pressed into aperturesin the base sheet;

FIG. 5 is an end elevation of the means of FIG. 4;

FIG. 6 is a section on line A-A of FIG. 3 through the stencil supportand mounting means;

FIG. 7 is a plan of the whole of the metal stencil partly shown in FIG.1;

FIG. 8 is an enlarged fragmentary detail of the supporting andtensioning frame and comprising a perspective view of a corner regionwith part of the stencil illustrated—the displacing means being in theinflated condition (the other corners correspond except for inlet 21);and

FIG. 9 is a view similar to FIG. 8 but with the displacing meansdeflated and the consequential movement exaggerated.

DETAILED DESCRIPTION

The apparatus illustrated in FIGS. 1, 2, 8 and 9 comprises a squaresupport and tensioning frame member 1 formed of channel sides 3 ofsubstantially U-shaped section with inclined corners joined byconnecting blocks 22. A square stencil 2 (only a quadrant/quarter 5thereof being illustrated in the upper left-hand part of FIG. 1) istensionable on the frame 1 by four rows of teeth 7, each extending froma mounting plate 7′ (only one shown) mounted on an inclined surface 4Aon each of the displacement mounting bars 4 and held in position bybolts 7″ extending therethrough and in to the bars 4. The teeth 7 arelocatable in the elongate slots 6 in the four sides of the stencil toengage at the ends thereof to tension the stencil 2. The teeth extend at45° to the vertical. The slots 6 and separating metal strips 6′ are ofsubstantially equal width so that approximately 50% of the stencilmaterial is removed to facilitate flexing and even tensioning.

In FIG. 1, the bottom of the apparatus is illustrated; i.e., it isviewed from below in the position of use and also of mounting anddismounting. The stencil at least in its central region is the lowermostsurface of the apparatus and normally has apertures (not shown) formedtherein for the passage of solder paste or other material—the thicknessof deposition of which is determined by the thickness of the stencilsheet, which is normally of metal.

Four elongate displaceable mounting bars 4 are provided with each onebeing reciprocably and linearly displaceably mounted on slider bars 8 atleast extending through end mounting blocks 4B between the walls (10,11) defining the channel sections 3 and a plurality of compressionsprings 9 are provided along each side and being located in bores 1′ inthe frame 1 and bores 4′ in the bars 4 for biasing the mounting bars 4into an outwardly displaced position in which the teeth 7 carriedthereby in use engage in the apertures 6 along the flexed sides of thestencil 2 and act to tension the stencil, with the outer edge regions ofthe stencil 2 being flexed out of the operative, lowermost main plane ofthe stencil sheet (over surfaces 10′)—the teeth 7 also being locatedabove said plane. The plurality of springs 9 are provided along eachside to provide for balanced tensioning.

The normally inverted U-shaped channel section 3 illustrated in FIG. 2(and FIGS. 8 and 9) comprising two limbs 10 and 11 connected by a web 12and has an inner limb 10 with an elongate stencil support surface 10′which can be curved to provide a smooth transition and is locatable inposition lower than the surface 17 of a stencil support 13 (FIGS. 3-6 tobe described) and is in such disposition above the surface 11′ of theouter limb 11 of the U-shaped section, which surface 11′ is intended torest on the upper surface base member (14) of a stencil support means13. An inflatable, flexible tube 20 (shown compressed/deflated in FIGS.2 and 9) extends around the frame between outer limbs 11 and bars 4 andalso extends from inlet pipe 21, which leads to a pressurized air supplyto effect inflation and control valve means, which also releases thepressure within the flatable tube/airbag 20. The other end of tube 20 isclosed at the corner block to which inlet pipe 21 is connected. The tube20 extends between bars 4 and the outer limb 11 of frame 1. Upinflation, the tube urges bars 4 against springs 9 to permit stencilloading/unloading. An L-shaped retaining plate 23 is secured along eachside, and one limb thereof holds the tube 20 in place—see FIGS. 8 and 9.Corner blocks 22 have passages (not shown) through which tube 20 extendsto its closed end and thus simultaneous inflation or deflation ispossible along the four sides.

The stencil supporting and mounting means 13 are illustrated in FIGS. 3to 6 and comprise a square base panel 14 of still material and fouridentical and identically located projections 15 extending therefrom andparallel to the four sides of base panel 14, which has three aperturesin each side for securely receiving locating lugs 16. Each projection 15has support surfaces 17 for receiving the outer edges of the stencil 2and a guide surface 18 against which the inner surface of limb 11 of theframe 1 slides. The four surfaces 18 together act to precisely locatethe frame 1 relative to the apertures 6 in stencil 2 so that teeth 7enter such (i.e., are in register).

In an alternative arrangement (not illustrated), four identicallyarranged but partly inclined slotted projections are provided to providethe same support for the stencil; but, additionally, the slots registerwith the slots of the stencil when laid on top to facilitate theengaging/disengaging of the teeth. Locating bolt heads also locate inapertures in the corners of the mounting frame so that accurateregistration is achieved.

In explanation of the cooperation of the frame 1, stencil 2 and mountingmeans 13 of FIGS. 3-6, the upper surface 17 and vertical surface 19′ ofall four locating projections 15 act to precisely locate the frame inposition with the inner shoulder 19′ also precisely locating the stencilso that, when the stencil 2 is located between the four projections 15,it is precisely located; and then the supporting tensioning frame 1 islowered on to the mounting means 13 so that the surface 10′ downwardlydisplaces the central portion of the stencil 2 with the outer edge 2′being supported on the shoulder 17 so that, as the tensioning frame 1 islowered, the teeth 7 on each of the mounting bars 4 enter into theapertures 6 in the side regions of the stencil 2.

Once the mounting frame has been located with the teeth 7 extendingthrough the elongate apertures 6 in the edge regions of the stencil, anoptionally operable displacing means or inflated inflatable tube 20 isde-pressurized (via outlet pipe 20 to assume the position of FIG. 9)with the result that the spring biasing means 9 urge the mounting bars 4and teeth 7 of all the bars to move outwardly such that the teeth 7cause the stencil 2 to be tensioned in an accurate and easy manner oversurface 10′. The teeth 7 lie out of the main plane of the stencil in thetensioned condition thereof and at all times. In other words, the teeth7 terminate at a shorter distance from web 12 than does the stencilsupport wall 10 with surface 10′.

The plates 7′, from which the rows of teeth 7 extend, are removable ifdesired by suitable disengagement of threaded bolts 7″, which abutagainst an inclined surface 4A extending along a side of the bar 4.Shims 7A are mountable in a similar manner.

The apparatus has considerable advantage in that the support means maycomprise the packaging of a foil or stencil, e.g., of cardboard or thelike; and, thus, no handling of the stencil is required for mounting onthe support frame or for dismounting, as such may occur on thesupporting means, which provides for considerable safety and avoidsdamage. Additionally, the springs maintain the stencil in tension whenin use and enable the provision of a strong even pressure, and bowing ofthe side walls is avoided or minimized.

The stencil support and mounting means comprises spaced-apart supportand locating means for supporting and locating the opposite edges of arectangular stencil above a space into which the main body of thestencil is downwardly flexible. Guide and locating means for guiding andlocating a rectangular supporting and tensioning frame for the stenciland associated with said support and locating means such that themounting teeth of said frame register with the apertures in the stencilwhen the stencil and frame are supported on the support and mountingmeans.

1. An apparatus for printing materials onto surface contacts on anelectronic substrate, the apparatus comprising: a stencil comprising ametallic sheet having one or more apertures formed through the thicknessof the metallic sheet to define a pattern for printing solder on asubstrate, the metallic sheet having four peripheral edges, and themetallic sheet defining more than two slots along each peripheral edge;a support frame having four side members with each side member connectedto two other side members and including a channel that extends alongeach side member; at least first and second displaceable opposedmounting members, with each mounting bar disposed within the channel andincluding a mounting plate that carries a plurality of projectionsextending from the mounting plate to comprise a terminal side edge, theprojections configured to engage the slots along a peripheral edge ofthe metallic sheet; a mechanism to bias the first mounting bar away fromthe second opposing mounting bar; and an inflatable tubular memberdisposed within the channel and extending along the four side members todisplace the first mounting bar toward the second opposing mounting barwhen the tubular member is inflated.
 2. The apparatus of claim 1,wherein the stencil is mounted on and held in tension by the projectionsof the mounting members.
 3. The apparatus of claim 1, wherein thechannel comprises a substantially U-shaped section with an elongateouter wall and an elongate inner wall, wherein the inner wall is ofgreater length and height than the outer wall.
 4. The apparatus of claim3, wherein the inner wall comprises an elongate stencil support surfacethat the stencil abuts when the stencil is mounted to the support frame.5. The apparatus of claim 4, wherein the elongate stencil supportsurface is curved to facilitate flexing of the stencil.
 6. The apparatusof claim 3, wherein the mechanism to bias the at least first mountingbar away from the at least second opposing mounting bar comprises aplurality of compression springs disposed along the inner wall of thechannel adjacent each mounting bar which are received by a plurality ofbore holes disposed along the mounting plate to accept the plurality ofcompression springs.
 7. The apparatus of claim 1, wherein the pluralityof projections does not extend into a horizontal plane of the stencilwhen the stencil is engaged and flexed thereon.
 8. The apparatus ofclaim 1, wherein the mounting plate is removably attached to theinclined surface by a plurality of fasteners disposed along the inclinedsurface.
 9. The apparatus of claim 1, wherein the inflatable tubularmember further comprises an air control mechanism connected to a firstterminal end of the inflatable tubular member to control a supply of airto the inflatable tubular member.
 10. The apparatus of claim 9, whereinthe air control mechanism includes an air inlet pipe with a terminal endremovably connected to an external air supply and a release valvemounted to the air inlet pipe to control flow of air.
 11. The apparatusof claim 9, wherein the inflatable tubular member further comprises asecond terminal end that is closed.
 12. The apparatus of claim 1,wherein the side members of the support frame form a substantiallyorthogonal structure.
 13. The apparatus of claim 1, wherein a firstmounting member is engaged with a first side member of the frame, asecond mounting member is engaged with a second side member of theframe, a third mounting member is engaged with a third side member ofthe frame, and a fourth mounting member is engaged with a fourth sidemember of the frame.
 14. The apparatus of claim 1, wherein oppositeedges of the metallic sheet are of equal length.
 15. The apparatus ofclaim 1, wherein the four peripheral edges of the stencil each definemore than twenty slots.
 16. The apparatus of claim 1, wherein the slotsare in the form of elongated slots separated by elongated strips. 17.The apparatus of claim 16, wherein the elongated slots and the elongatedstrips are of substantially the same width.
 18. An apparatus forsupporting and tensioning a stencil to enable solder printing of circuitboards and other electronic substrates, comprising: a support framehaving four side members with each side member connected to two otherside members and including a channel that extends along each sidemember; at least first and second displaceable opposed mounting barswith each mounting bar disposed within the channel; a mechanism to biasthe at least first mounting bar away from the at least second opposingmounting bar; and an inflatable tubular member disposed within thechannel and extending along the four side members to displace the atleast first mounting bar toward the at least second opposing mountingbar when the tubular member is inflated.
 19. The apparatus of claim 18,wherein each mounting bar includes a mounting plate that carries aplurality of projections extending from the mounting plate.
 20. Theapparatus of claim 18, further comprising a stencil mounted on themounting members.